This invention relates to straddle type mobile cranes, and more particularly, to a steering system for such cranes.
Straddle type mobile cranes are commonly employed for lifting and moving large bulky objects, such as shipping containers, structural members, modular building sections, heavy machinery and equipment, and the like. Such devices are well known and generally include a pair of inverted U-shaped gantries comprising cross beams spanning vertical columns which are supported at their lower ends on wheels or truck assemblies. The two-gantry assemblies are interconnected by spaced side members and may include hoists supported from the cross beams by means of a trolley so that the load may be elevated and transversed laterally.
On one type of mobile crane, each gantry includes front and rear wheels or trucks with at least the front wheels being driven by individual hydraulic motors. In addition, at least the front wheels are mounted on forks or similar structures so that they can be pivoted about a vertical axis to permit steering. Because of the distance between the wheels, the wheel at the outside of the turn must travel through a longer arc than the inside wheel. In order to permit this speed differential between the inside and outside wheels as the crane moves through a turn, the common practice was to interconnect the hydraulic systems of each wheel motor to provide a differential effect. With the hydraulic systems interconnected, fluid would tend to flow to the wheel which offered the least resistance to fluid flow so that the more rapidly turning outside wheel would receive a greater portion of the fluid flowing to both motors and the inside wheel would conversely receive less. Such differential systems are normally satisfactory for turning but are unsatisfactory should the wheels encounter surfaces having different resistances. For example, should the vehicle engage an ice pack, one of the wheels would begin to spin. Under these circumstances, most of the hydraulic fluid would flow to the spinning wheel so that the device would become stalled. Also, should one of the wheels engage an obstacle, a greater quantity of fluid would flow to the other motor thereby causing the crane to veer toward one side.